The most commonly used apparatus for pressure leaching ore is a multi-compartment autoclave. Multi-compartment autoclaves are typically constructed of a single pressure vessel divided into multiple chambers. In the multi-compartment autoclave, a feed slurry is gravity fed from chamber to chamber to progressively leach metal values from the ore. The lixiviant for lateritic ores is typically sulfuric acid, which is added to the first chamber and possibly to additional chambers of the autoclave. A schematic diagram of a multi-compartment autoclave used for pressure leaching of lateritic ores is provided in U.S. Pat. No. 4,374,101. The multi-compartment autoclave provides the advantages of efficient use of space and relatively low cost. However, when a single chamber of a multi-compartment autoclave requires maintenance, the entire autoclave must be shut down for service. This interruption of pressure leaching significantly limits leach liquor production from lateritic ores.
Alternative plant designs have relied upon a cascade of single compartment autoclaves. The single compartment autoclave design is more flexible to operation upsets. For example, provided the single autoclaves have the required valve and piping configuration, continued operation of functional autoclaves is possible while servicing an autoclave. However, the disadvantages of the single compartment autoclave design include increased cost and inefficient use of plant space.
The acidic leaching of lateritic ores with sulfuric acid requires temperatures of about 240.degree. C. or higher to proceed sufficiently rapidly to be of industrial interest. Unfortunately, leaching is accompanied by the formation of scale trader these conditions. The scale consists mainly of hematite (Fe.sub.2 O.sub.3), hydronium alunite (H.sub.3 OAl.sub.3 (SO.sub.4).sub.2 (OH).sub.6) and basic iron sulfate (FeSO.sub.4 OH). The scale covers all surfaces exposed to the leach slurry, i.e. the autoclave walls and internals.
Scale formation is generally the result of super-saturation with respect to specific dissolved species. Super-saturation may, for example, be caused by rapid temperature changes and when the precipitation reaction is kinetically slow. In the case of laterite ore leaching, iron and aluminum are present in the ore mainly as goethite (FeOOH) and gibbsite (Al(OH).sub.3), respectively. The iron and aluminum values originally contained in the ore dissolve due to reaction with sulfuric acid, and re-precipitate mainly in the form of hematite (Fe.sub.2 O.sub.3) and hydronium alunite (H.sub.3 OAl.sub.3 (SO.sub.4).sub.2 (OH).sub.6), respectively. Essentially all of the precipitates report to the leach residue. However, since the precipitation reaction is relatively slow with respect to the dissolution reaction, this leads to super-saturation which results in scale formation.
Most of the scale forms in the compartments to which acid is added, i.e. where the iron and aluminum concentrations are relatively high. If the acid is added only to the first chamber, then most of the total scale found in a multi-compartment autoclave will be produced in the first compartment. When the rate of scale formation in the first compartment is rapid, the entire multi-compartment autoclave must be shut down frequently for descaling. In order to even out the rate of scale formation in a multi-compartment autoclave used for the acidic leaching of lateritic ores, Fekete et al., in U.S. Pat. No. 4,098,870, disclose incremental acid addition to several compartments. The acid is added incrementally in order to limit the iron and aluminum concentrations to less than about 4 grams per liter. While this process effectively reduces the rate of scale formation in the first compartment of a multi-compartment autoclave, it does require an increased retention time for adequately leaching the contained nickel and cobalt values.
Several methods have been proposed over the years to control scale formation in autoclaves used to leach lateritic ores. Opratko et al., in U.S. Pat. No. 3,809,549, disclose pressure leaching at a temperature between 200.degree. and 260.degree. C. with pyrite and oxygen for the "in situ" generation of sulfuric acid. Iler et al., in U.S. Pat No. 4,399,109, disclose a method of recycling slurry to an intermediate temperature flash stage to limit silica scale formation. In another approach for controlling the scale formation during acidic leaching of lateritic ores in an autoclave, Queneau et al., in U.S. Pat. No. 4,415,542, disclose the leaching of lateritic ores with sufficiently high magnesium contents (3% to 30% by weight) to inhibit the formation of hematite/alunite scale while favoring the formation of a scale rich in magnesium sulfate which is water soluble. Finally, Sherwood et al., in U.S. Pat. No. 4,627,900, disclose a method of minimizing scale formation by periodically reversing current between an autoclave and an electrode submerged within the autoclave. As far as known, none of the above methods has completely eliminated the requirement for periodic scale removal from lateritic autoclaves.
Typically, the scale may be removed by shutting down an autoclave and dissolving the scale with a strong acid or physically chipping away pieces of scale. For example, Lussiez et al., in U.S. Pat. No. 4,374,101 ('101), disclose lowering the autoclave temperature to about 200.degree. C. and dissolving the scale deposits with about 30 to 40 g/l H.sub.2 SO.sub.4. This procedure, requiring about 60 hours for a 93 percent removal of hematite-alunite scale, is quicker than the five to seven day physical removal process presently used at the Moa Bay operation in Holquin Province, Cuba. The problem with the process of the '101 patent is that it also requires the autoclave to be shut down for several days during cleaning operations.
It is an object of this invention to provide a pressure leaching autoclave configuration for lateritic ore that may essentially be operated on a continuous basis, i.e. with which less frequent stoppages are required for descaling.
It is a further object of this invention to provide a method for operating a multi-compartment autoclave on a continuous basis for pressure leaching of lateritic ores.
It is a further object of the invention to provide an autoclave configuration that allows flexible operating conditions for pressure leaching of lateritic ores.
It is a further object of this invention to provide an autoclave that maintains the flexibility of single-compartment autoclaves without the loss of plant floor space and increased costs associated with single-compartment autoclaves.